The present invention relates in general to audio systems which prevent distortion from amplifier clipping by employing voltage limiting, and more specifically to automotive audio systems in which power amplification of an audio signal is performed in a location remote from a main audio unit.
Typical audio reproduction systems include a variable-gain preamplification stage followed by a fixed-gain power amplifier which drives an output transducer such as a speaker. A volume control voltage provided to the variable-gain stage controls the output volume heard by a listener.
An important objective in designing an audio system is to provide minimum distortion in signal reproduction. However, there is always some distortion, especially at high sound levels. As the magnitude of the signal provided from the variable-gain preamplifier stage to the power amplifier increases above a certain level, the power amplifier becomes overdriven. This situation occurs when the input signal to the power amplifier multiplied by the fixed-gain of the power amplifier approaches the supply voltage provided to the power amplifier. As a result, the power amplifier becomes saturated and the signal peaks of the audio signal are distorted by clipping. The variable-gain preamplifier is also susceptible to clipping, but the power amplifier is likely to begin clipping first.
The problem of clipping is severe in automotive audio systems. Less voltage headroom (i.e., safety margin) is available to the power amplifier since the automobile is limited to a 12-volt electrical supply. Although a DC/DC converter can be used to obtain a higher DC voltage, such converters are relatively expensive. Also, bass boost is needed in the automotive environment to mask low frequency road and engine noise, making clipping more likely.
High performance automotive audio systems typically employ a power amplifier located remote from the main audio unit in the dashboard, where it is not visible to the occupants of the automobile. A remotely located power amplifier saves space in the dashboard and reduces the amount of heat that would otherwise be generated in the main unit.
When the two amplifier stages are not located together, the problem of clipping of each amplifier stage has been handled individually in the prior art. Therefore, either one or both amplifier stages have been provided with voltage limiting in order to reduce distortion from signal clipping.
In a voltage limiting amplifier, whenever the amplifier output exceeds a predetermined referenced voltage, the input signal to that amplifier stage is attenuated in order to ensure that the amplifier is not overdriven. In the prior art remote power amplifier systems, unless both the variable-gain amplifier in the main unit and the power amplifier in the remote unit are separately provided with voltage limiting, clipping distortion can result at sufficiently high input signal levels or gain levels, and once distortion is introduced into the signal it cannot be removed.
An audio system employing voltage limiting amplifiers in both the main variable-gain preamplifier and the remote power amplifier avoids clipping distortion, but at the expense of an additional source of distortion referred to as modulation-type distortion. The gain reduction elements employed in the voltage limiting feedback circuits necessarily have time constants characteristic of their operation. Specifically, there are attack times and release times which govern the gain reduction. It is relatively difficult and expensive to provide perfectly matched attack and release times in the two independent limiters acting in series, and any mismatch in attack and release times creates distortion in the audio signal.